Conversion of heat energy into electrical energy (i.e., a heat engine/generator) is highly desirable for space missions, for example, to convert the heat of radioisotope decay into power for the instruments of spacecraft. It is also desirable for commercial uses, particularly if the conversion devices are small and low cost, and even more so if they can be enabled to work on waste heat from other processes. For space mission applications, it is further desirable that the conversion device be functional in a vacuum ambient environment. In order to take advantage of waste heat, the heat engine/generator should have a good conversion efficiency when utilizing relatively low temperature and relatively small scale heat sources. Furthermore, the device should be mechanically simple and maintenance free to promote reliable long-term operation.
U.S. Pat. No. 4,733,121 (Hebert; 1988), discloses a solid state heat to electricity converter wherein the conversion of heat energy to electric energy is realized by coupling memory materials to piezoelectric materials or to composite magnetoelectric materials and by subsequent hot-cold-strain cycling of the memory material. The embodiment of Hebert's FIG. 1 incorporates memory material in the form of a wire (12) wrapped around an assembly of piezoelectric or magnetoelectric plates (13) such that when the wire (12) is electrically self-heated it contracts, thereby compressing the plates (13) to generate electric output. Hebert's FIG. 3 illustrates a tubular memory material (23) surrounding washers (26) of piezoelectric or magnetoelectric material. Hebert's FIG. 4 illustrates a “bender” wherein a U-shaped foil (31) of memory material is bonded to parallel wafers (28) of piezoelectric or magnetoelectric material. Hebert's FIGS. 5–7 illustrate an embodiment of said benders (36) embedded around a disk (35). The benders can be heated by solar radiation (R) or a combustible gas (G), and cooled by a cold liquid (C) as the disk (35) rotates. A magnet (37) fixed near the station where the benders are heated can provide motive force for a heat engine implementation.
Small scale, low cost devices can be fabricated using micro-electromechanical system (MEMS) technologies. Micro-electromechanical (MEM) devices are formed by well-known semiconductor processing techniques such as etching and photolithography. MEMS can be formed from semiconductor materials, such as single crystal silicon wafers, or from polycrystalline silicon. MEMS fabrication techniques can also be used on other materials, such as silicon carbide or glass. Typical size scales of MEM devices are micrometers to millimeters in scale, with some of the smallest dimensions occasionally less than a micrometer (micron, or μm) in size. MEM engines are thus much smaller than conventional engines. Because of the small size, many devices can be formed on a single wafer. For example, hundreds or thousands of individual heat engines could be formed on a single thin wafer.
U.S. Pat. No. 6,363,712 (Sniegowski, et al.; 2002), discloses a gas-driven microturbine fabricated by a three-level semiconductor batch-fabrication process based on polysilicon surface-micromachining. This provides a good example of MEMS technology applied to machines with moving parts.
U.S. Pat. No. 6,275,325 (Sinclair; 2001), discloses a thermally activated micro-electromechanical systems (MEMS) actuator having coupled members that undergo different amounts of thermal expansion for moving micromechanical objects, such as a mirror. The actuator members comprise first and second elongate members (224, 226). When current is applied to resistive electrical paths on the actuator members, movement results from thermal expansion that is greater in the second member than in the first member.
Heat engines have long been used to generate electricity, typically by heating/cooling a motive medium (e.g., combustion heating of a gaseous medium), and then using the expanding/contracting motive medium to create mechanical energy by moving mechanical parts (e.g., piston, or turbine) in a heat engine. The mechanical output of the heat engine (e.g., linear movement or rotation) is then coupled to a generator to convert the mechanical energy to electrical energy (e.g., moving wires in a magnetic field, or flexing piezo-electric elements). Many well known types of heat engines utilize internal combustion (e.g., Otto engine or Diesel engine) wherein the motive medium is an air/fuel mixture that is combusted in a chamber. The expanding gases resulting from the combustion are typically used to drive a piston or a turbine. Such engines are relatively complicated, requiring means for mixing fuel and air, and valve means for admitting fuel/air and for exhausting combustion byproducts. Nevertheless, the prior art discloses a number of internal combustion heat engine embodiments fabricated as MEM devices, many of which are then coupled with electric generation means.
U.S. Patent Application Publication 2002/0148237 (Thiesen, et al.; 2002), discloses miniature reciprocating heat pumps and engines, i.e., a miniature thermodynamic device, that can be constructed using standard micro-fabrication techniques (MEMS). An embodiment relates to generation of electrical power wherein a reciprocating piston works against either an electrostatic or a magnetic field. Thiesen's FIGS. 6a–6d illustrate a heat engine/generator operating as a reciprocating internal combustion engine comprised of: piston (10), piston housing (20), metal layers (40, 41) formed on the piston and metal layers (50, 51) formed on the piston housing to provide a capacitor between the piston and the piston housing. Inlet manifolds (221, 224) and inlet valve structures (211, 214) isolate the inlet fluid streams and meter the fuel source. An exhaust manifold (223, 225) and exhaust valve structures (212, 213) allow for the removal of the combustion products. Circuitry for the collection, storage, and distribution of electrical energy generated by the capacitor plates may also be provided, as known in the art. As an alternative to internal combustion, hot gases from external combustion (or any other source of heat that raises the temperature of a gas) may be admitted through the inlet valves for expansion in the chamber against the reciprocating piston.
U.S. Pat. No. 6,109,222 (Glezer, et al.; 2002), discloses miniature reciprocating combustion-driven machinery implemented using micromachining technology wherein a micro heat engine (10) uses a reciprocating free piston (11) driven by a periodic combustion process that alternates combustion between combustors (20, 21) at opposed ends of the piston (11). The combustors comprise suitable means for introducing air and fuel (e.g., valve 25), and means for igniting a combustible mixture (e.g., spark plug 26). As described in column 5, with reference to Glezer's FIGS. 4A and 4B, a preferred embodiment for converting the motion of the piston (11) to electrical energy operates on the principle of magnetic commutation wherein the piston is a rotor of a linear generator and has magnetic regions (32, 33, 34, 35); and the stator (31) comprises permanent magnets (45, 46, 47) alternating with conductor-wound teeth (42, 43, 44) of a back iron (41).
U.S. Pat. No. 6,276,313 (Yang, et al.; 2001), and U.S. Pat. No. 6,397,793 (Yang, et al.; 2002), disclose a microcombustion engine/generator constructed in three layers of micromachined material. The middle layer has two linear free pistons and vents for directing gases and fuels into and out of a central combustion chamber. Electrical energy can be generated by means of permanent magnets (34, 35) in the pistons (21, 22) that move in the fields of electromagnets (36, 37).
The Stirling engine is a well known type of heat engine in which a fixed amount of gas (e.g., hydrogen or helium) is compressed in a cold chamber. The gas is then transferred to a hot chamber, which is heated by an external heat source (typically external combustion), where the gas expands and drives a piston, providing mechanical energy that delivers work. Then the gas is returned to the cold chamber, where it is cooled and the cycle begins again.
Although not implemented in MEMS, an example of a Stirling heat engine/generator is seen in U.S. Pat. No. 4,511,805 (Boy-Marcotte et al.; 1985), that discloses a converter for thermal energy into electrical energy using Stirling motor and integral electrical generator. The machine is completely sealed, having a power piston (15) that drives a linear alternator (20, 21, 22). A displacing piston (7) travels within a cavity (5) causing circulation of the working fluid (e.g., helium) through a circuit (1) that communicates with chambers (5a, 5b) formed on either side of the movable displacing piston (7). The circuit (1) comprises successively: a hot heat exchanger (2), a regenerator (3), and a cold heat exchanger (4).
It is an object of the present invention to provide reliable, maintenance free, low-cost, small scale, heat driven electrical energy generation. It is a further object to meet the objectives with a simple heat engine that does not require the complexities of prior art engine/generators with fluid control valves and complex moving parts subject to wear and breakdown. Further objects include utilization of various types of waste heat, and the ability to function in a vacuum.